The present invention relates to apparatus for detecting the passage of a moving body at a determined point on its guided displacement along a track. A particularly important application of the invention lies in automatically controlled public transport installations.
Such detection apparatus is already known (EP-A-0 570 289 or U.S. Pat. No. 5,451,941) comprising an interrogation assembly carried by the moving body and designed to be connected to an electrical power supply, and a passive responder associated with the track, and in which:
the interrogator assembly has a low frequency transmitter and a medium frequency transmitter both operating continuously while the moving body is moving, each having a respective antenna emitting towards a zone which is determined relative to the antennas and through which the responder passes during displacement; and
the responder comprises a low frequency signal receiver circuit and a medium frequency signal receiver circuit having respective tuned radiating circuits associated in such a manner that a low frequency signal induced in the low frequency circuit by passage through the zone causes the medium frequency radiating circuit of the responder to be closed at the rate of the low frequency, with the interrogation assembly also having means responsive to disturbances in the medium frequency transmitter caused by closing the medium frequency tuned radiating circuit of the responder when the responder is in said zone.
A public transport installation generally includes a very large number of responders distributed along its tracks and frequently referred to as "beacons". Even when maintenance is performed regularly, it is always possible that a beacon will break down. Such a breakdown is generally due to failure of an active electronic component, rather than to a break in a passive element such as a resonant circuit. Although the breakdown of one of the responders along the ordinary portions of the track is of little consequence, the same is not true of a responder situated in a station. Station responders are often used not only to detect the passage of a moving body, but also to verify that the moving body has indeed come to rest in a well-determined location, for which location the responder is in the above-mentioned zone. If a responder delivers no response when the moving body passes through a station, often the only possible solution is to omit any stop of the moving body in that station, particularly when the platforms are closed off from the track by a wall provided with doors that open only when the moving body is stationary and in a well-determined position.